This invention relates generally to improvements in knee prostheses, particularly with respect to an improved tibial component constructed from a relatively hard and relatively high strength ceramic material. The ceramic tibial component includes at least one generally articulation surface designed for direct bearing engagement by and articulation against an associated femoral articulation surface having a generally convex or condylar shape. The ceramic tibial component is design for ultra low wear over an extended service life, and further permits elimination of a conventional polymer-based bearing insert and undesirable wear debris problems associated therewith.
Knee prostheses generally comprise a tibial component adapted for fixation to an appropriately resected upper end of the patient's tibia. Such tibial components have generally been formed from a biocompatible metal material such as cobalt chrome, titanium, stainless steel, or from polymer-based materials. The tibial component thus provides a strong and durable prosthetic base structure for supporting a polymer-based bearing insert which in turn defines a pair of upwardly presented, generally concave bearing seats for articulatory engagement with a corresponding pair of generally convex or condylar-shaped articulation surfaces at the lower end of the patient's femur. These femoral articulation surfaces can be defined by natural femoral bone surfaces, or alternately by condyle surfaces on a reconstructed or prosthetic femoral component affixed to the patient's femur. Such femoral components have generally been formed from a biocompatible metal material such as cobalt chrome, titanium, stainless steel, zirconium, zirconium oxide and ceramic materials such as alumina, zirconia and zirconia-toughened alumina (ZTA).
The polymer-based bearing insert is formed typically from a high density or ultra high molecular weight polyethylene (PE) material, which has been shown in various specific compositions to accommodate smooth and relatively low-wear articulation relative to the femoral surfaces. However, clinical studies have shown that significant wear debris can be generated and released by the polymeric bearing insert over a period of time, and further that a principle contributing factor to implant failure is osteolysis attributable at least in part to the presence of such polymer-based wear debris. More particularly, such studies have shown that PE wear debris released into peri-implant tissues appears to elicit a deleterious biological reaction, incorporating foreign body giant cell and macrophage cell responses leading to undesirable bone resorption, with eventual loosening and failure of the prosthetic implant. As a result, alternative prosthesis constructions have proposed improvements in and to the polymer-based bearing insert, such as the use of highly cross-linked polyethylene materials. Other alternative prostheses have been proposed using rigid-on-rigid components, such as ceramic-on-ceramic or metal-on-metal, thereby eliminating the polymer-based bearing insert and wear debris associated therewith. Bearing couples in the knee joint have been limited to metal-on-polymer or ceramic-on-polymer.
In general, ceramic knee prosthesis components have shown promise for use in a ceramic-on-ceramic or alternately in a ceramic-on-metal articulating interface, thereby completely eliminating the polymer-based bearing insert. Such prosthesis constructions, when formed with a good surface finish and conformal surface geometry, have demonstrated a relatively low coefficient of friction and resultant substantial reduction in component wear in comparison with ceramic-polymer or metal-polymer articulatory interfaces. However, the major limitation on the use of ceramic components particularly such as alumina-based ceramic materials has been an unacceptably high rate of brittle fracture occurring within a post-surgical follow-up period ranging from a few months to several years. In this regard, ceramic materials generally exhibit relatively low toughness and are thus prone to brittle fracture.
U.S. Pat. No. 6,881,229 discloses an improved ceramic material for use in joint prostheses, such as knee prostheses, wherein a ceramic-on-ceramic or a ceramic-on-metal articulatory interface is defined. The improved ceramic material comprises a doped silicon nitride (Si3N4) having relatively high hardness, tensile strength, elastic modulus, lubricity, and fracture toughness. Specifically, the improved doped silicon nitride ceramic has a flexural strength greater than about 700 Mega-Pascal (MPa) and a fracture toughness greater than about 7 Mega-Pascal root meter (MPam0.5). This high strength and high toughness doped silicon nitride ceramic achieves ultra-low wear over an extended service life, with dramatically reduced risk of brittle fracture.
In addition, U.S. Pat. No. 6,846,327 discloses improved ceramic materials for bone graft applications, wherein the ceramic material is designed to mimic structural characteristics of natural patient bone by including first and second regions of comparatively lower and higher porosity to respectively mimic natural cortical and cancellous bone structures. The preferred ceramic materials disclosed exhibit a flexural strength greater than about 500 Mega-Pascal (MPa) and a fracture toughness greater than about 5 Mega-Pascal root meter (MPam0.5). In use, the relatively low porosity region of the ceramic material provides high structural strength and integrity, whereas the higher porosity region is suitable for bone ingrowth to achieve secure and stable implant affixation.
The present invention comprises an improved knee joint prosthesis particularly wherein the load-bearing tibial component thereof is constructed from an improved high strength and high toughness ceramic material as disclosed, e.g., in U.S. Pat. No. 6,881,229 and/or U.S. Pat. No. 6,846,327.